According to the trend of recent years for a high performance and a high signal-processing rate, there has been enhanced a demand for making the size of the wiring substrate smaller, the pitch of the wiring pattern layers finer and the radius of the via conductors smaller.
For example, an insulating resin layer between one wiring pattern layer and two adjacent wiring pattern layers is generally restricted by a practical limit of the section of a length×a width of 25 μm×25 μm. However, it has been demanded that the length and the width are individually 20 μm or less.
In order to satisfy these demands, it is necessary not only to reduce the diameters of the via conductors for connecting the wiring pattern layers on the upper side and on the lower side and the via holes for fitting the via conductors.
In case the lower lands have a diameter of 132 μm and the upper lands have a diameter of 138 μm, for example, the via holes and the via conductors formed inbetween in the prior art has a lower end diameter of about 60 μm and an upper end diameter of 68 μm (as referred to JP-A-2003-258430 (page 4, FIGS. 1 and 2), for example).
In case the wiring pattern layers are formed into a finer pitch such as the lower layer land having a diameter of 103 μm and the higher layer land having a diameter of 110 μm, it is necessary to confine the diameter of the lower end of the via holes and the via conductors to be formed between those lands within a range of about 40 to 60 μm and the diameter of the upper end of the same to about 65 μm or less.
However, it has been impossible for the insulating resin layer of the prior art containing about 18 wt. % (% by weight) of the inorganic filler under the laser irradiating conditions (e.g., the output: 0.4 mJ, the aperture diameter of the slit opening of a laser machining apparatus: 0.5 μm, the shot number: 2) to form the via holes having the aforementioned small diameter in the insulating resin layers.